The effects of energy and crude protein (CP) supplementation to lactating ewes on the performance of the ewes and their lambs, while grazing native pasture (veld), were studied during two autumn (1981 and 1982) and two spring (1983 and 1984) lambing seasons. Different levels of energy and / or CP were provided daily via rumen fistulae to the ewes. Body mass loss of lactating Merino and Dorper ewes was not reduced by either supplementary energy or CP during the autumn / winter (1981 and 1982), whilst the growth rate of their lambs was slightly improved. Body mass loss of ewes was reduced by 50% and growth rate of their lambs improved markedly by combined supplementation of energy plus CP. Irrespective of treatment, Dorper ewes lost 100% more body mass during lactation than Merino ewes, but their lambs gained 100% faster than Merino lambs. In contrast, provision of energy and CP supplements (1983), or incremental levels of energy (1984) during the spring / summer to lactating Dorper ewes, had no definite effect on body mass changes of the ewes or growth rate of their lambs. The superior animal performance on veld during the spring / summer, in comparison to the autumn / winter, is discussed and put into perspective.